1. Field of the Invention
The present invention relates to a method of manufacturing an object of a powdered material by isostatic pressing of a body preformed from the powdered material.
2. The Prior Art
When the preformed powder body is subjected to the isostatic pressing at the sintering temperature in order to provide a desired dense, sintered product, the body is enclosed in a casing which, during the pressing, may prevent the pressure medium used in that connection, normally a gas, from penetrating into the powder body. The casing as well as its contents are normally freed from undesirable gases during some process stage prior to the sealing. Different ways of forming the casing are known. According to one known method, a preformed capsule of glass is used as casing. According to another known method, the casing is manufactured on the spot by dipping the preformed powder body into a suspension of particles of glass, or in some other way surrounding it with a layer of particles of such glass and then heating it under vacuum at such a temperature that the particles form a dense casing around it. Possibly, two such layers of glass having different melting points are used.
Canadian Pat. No. 1,148,772 describes one method of manufacturing a dense casing in the form of a melt of glass according to which the preformed powder body embedded in a compound of glass particles is placed in a heat-resistant vessel and the compound is transformed into a melt, below the surface of which the preformed body is located when a pressure necessary for the isostatic pressing is applied on the melt by means of a gas.
The above-mentioned Canadian patent mentions a glass containing boron oxide as a suitable material for achieving a casing for a preformed body of silicon nitride since, when using such glass, it has been found to be possible to avoid a penetration of molten glass into a preformed body of silicon nitride. As a probable explanation of the fact that boron-containing glass does not penetrate into the silicon nitride body, there is mentioned the formation of a boron-nitrogen compound, probably boron nitride, in the boundary surface between the glass and the silicon nitride, before the glass forms a low-viscous melt, and that this boron nitrogen compound counteracts the penetration of the glass into the pores of the powder body. The content of B.sub.2 O.sub.3 in the glass is stated to amount to between 2 percent by weight and 70 percent by weight.
After the isostatic pressing, the finished object, after having been allowed to cool, is embedded in the used casing of glass which has then solidified. The casing normally has to be removed by blasting in order for the object not to be damaged during removal. In certain cases it may be possible to remove the main portion of the casing by heating it to a temperature at which the glass becomes sufficiently thinly fluid to run off and only leave a thin film which is removed by blasting. Also pickling may be used. The removal of the casing is time-consuming and--particularly in case of objects having complicated shape--an extremely troublesome operation.